1. Field of the Invention
The present invention relates to a pump system having a pump, including a centrifugal pump; and more particularly to a method for determining pump flow without the use of traditional sensors.
2. Brief Description of Related Art
Pumping devices known in the art, techniques associated with the same, and their shortcomings are as follows:
Controllers for pumps are known to use the Pump Affinity Laws, which are approximations of how the performance (flow, head, power) of a centrifugal pump is affected by speed and by impeller trim. While the affinity laws are effective for general estimations, the factoring coefficient for power frequently results in an over or under estimation of power based upon the operating speed, size and specific speed of the pump. This inaccuracy directly influences algorithms for pump protection and flow prediction that can be found in Programmable Logic Controllers (PLC), Distributed Control Systems (DCS), and Variable Frequency Drives (VFD).
Furthermore, when creating pump performance maps, variations in actual pump performance from standard performance curves significantly degrades the accuracy of flow and/or pump condition estimation. The most common solution to this is to perform a pump performance test at multiple speeds to confirm accurate pump performance. However, this solution can become timely, application specific and quite costly. In view of this, there is a need in the industry for a technique that overcomes the error of the affinity laws.
U.S. Pat. No. 6,715,996 B2, issued to Moeller, discloses a method for the operation of a centrifugal pump that samples the pump power at closed valve condition for two speeds, determines parasitic losses and calculates an adjusted power at other frequencies to determine if the pump is operating at closed valve condition. However, methods to correct power at closed valve condition like this begin to lose accuracy at speeds below 50% of nominal motor speed and can limit application range. The method of interpolation between power values at other speeds is based partly on the affinity laws and as such is less accurate.
PCT WO 2005/064167 A1 issued to Witzel, Rolf et al., discloses a technique that uses a calibrated power/differential pressure curve vs flow vs speed. The calibrated data is stored and compared to current values in order to determine pump flow. This technique requires a differential pressure transmitter and requires that calibration curves for power/Δ pressure vs. flow be stored in the evaluation device. This method is application specific to obtain flow thereby reducing flexibility during field setup. It is also not easily adjusted to compensate for wear.
U.S. Pat. No. 6,591,697, issued to Henyan, discloses a method for determining pump flow rates using motor torque measurements, which explains the relationship of torque and speed versus pump flow rate and the ability to regulate pump flow using a Variable Frequency Drive (VFD) to adjust centrifugal pump speed. However, this technique utilizes calibrated flow vs torque curves for several speeds which are application specific thereby reducing flexibility during field setup. It is also not easily adjusted to compensate for wear.
U.S. Pat. No. 6,464,464 B2, issued to Sabini, et al., discloses an apparatus and method for controlling a pump system based on a control and pump protection algorithm which uses a VFD to regulate flow, pressure or speed of a centrifugal pump. However, this technique requires the use of auxiliary instrumentation which adds cost and complexity to the drive system, a potential failure point, and unnecessary cost. It also utilizes calibrated Flow vs TDH curves at several speeds which are application specific thereby reducing flexibility during field setup.
Furthermore, the following patents were developed in a patentability search conducted in relation to the present invention. Below is a brief summary thereof:
U.S. Pat. No. 4,358,821 discloses a method and apparatus for the incorporation of varying flow in the control of process quantities, where the passing flow is measured and the amount of material flowed through the process is determined by integration of the results of the measurement.
U.S. Pat. No. 5,213,477 discloses an apparatus for pump delivery flow rate control, where maximum allowable flow is determined based on a relationship between the available and required net positive suction head (NPSH).
U.S. Pat. No. 6,424,873 discloses a method and system for limiting integral calculation components in a PID controller, based on a technique where an integral calculation component of a primary PID controller is excluded or a portion thereof or is included in a PID calculation.
U.S. Pat. No. 6,546,295 discloses a method of tuning a process control loop in an industrial process, where field device and process controllers are fine-tuned by determining control parameters for the controllers that interact to provide a desired process variability.
U.S. Pat. No. 6,554,198 discloses a slope predictive control and digital PID control for controlling a variable air volume (VAV) box in a pressure independent VAV temperature control system, based on a technique involving a calculation of an error between an airflow setpoint and measured airflow.
Patent Publication No. 2004/0267395 discloses a system and method for dynamic multi-objective optimization of machine selection, integration and utilization, based on a technique where asset utilization in an industrial automation system is modified based on a function of analyzed diagnostic and machine data.
Patent Publication No. 2005/0237021 discloses a rotatingly driving device of construction machinery, in the form of a method and apparatus for pumping a fluid at a constant average flow rate.
None of the aforementioned patents or publications teach or suggest the technique described herein for determining pump flow without traditional sensors.